Goals for the coming year: A. Studies of the distribution of axons of the S. gelatinosa using the horseradish peroxidase and the radioactive labelled leucine techniques. Both light and electron microscopic studies will be used and electron microscopic radioautography has been adopted and is working very satisfactorily in this laboratory. B. Investigation of the projections of marginal versus nucleus proprius neurons using both electro-physiological (antidromic firing) and labelling techniques (HRP and R.A. Leucine). It seems probable to this investigator, that the two types of neurons send their axons to different sites in the mesencephalon and thalamus; the above mentioned techniques either singly or in combination may resolve this issue which is of central importance in pain mechanisms. C. Studies of central distribution of regenerated dorsal root fibers. Using the radioactive leucine (or proline) techniques, the ganglia corresponding to dorsal roots sectioned three months to two years previously are injected and will be injected with 3 to 5 microns liter quantities of concentrated labeled protein. At 24 to 72 hours the animals are anesthetized and perfused and sections made for both light and E.M. radioautography. These are examined after three weeks to two months and in some instances up to 4 to 6 months. Background activity is determined by including sections of tissue which has had no radioactive label injected and processed at the same time as the test material. D. Studies of the excitatory and inhibitory inputs to marginal neurons using a combination of A and C fiber inputs separated by anodal polarization. A new technique for doing this with appropriate timing will be tested.